Level measuring methods of the type referred to above have been well-known from prior art, for instance as described in WO 01/11323 A1. Industrial applications often call for the determination of the level of a medium or substance such as a liquid or bulk material in a container, for instance a tank. A variety of techniques have been employed, differentiated between contact level measuring methods and non-contact level measuring methods. The contact level measuring methods in which a component of the measuring device touches the medium whose level is to be measured, include level determination by means of a float, a buoyant element or a sensor plate. Known contact level measuring methods further include capacitive measurements whereby the fill level is determined as a function of the capacitance between an electrode immersed in the medium and the wall of the container, as well as thermal measurements whereby the increased heat dissipation upon the immersion of a current-carrying, temperature-dependent resistance element in the medium is used as a measure of the electrical resistance that varies with and is indicative of the depth of immersion.
The non-contact level measuring methods include, for instance, measurements using a laser or ultrasound. A laser beam or ultrasonic signal is directed at and reflected back by the surface of the medium and the reflected signal is captured, with the runtime of the signal indicating the fill level of the medium. The same basic principle is employed in radar level measuring methods in which a microwave signal is generated and sent, via an antenna such as a rod antenna, a horn antenna or a patch antenna, in the direction of the medium whose level is to be determined and off whose surface it is retroreflected and recaptured by the same or some other antenna.
There exist several different radar level measuring methods. In the pulsed radar level measuring method, a microwave signal is transmitted in the form of short pulses, either unmodulated or carrier-frequency-modulated. The runtime of the microwave pulses from the transmitter to the surface of the medium and back to the receiver permits the determination of the distance between the transceiver and the medium, in which case one antenna can serve as the transmitter and the receiver. In the frequency modulated continuous-wave (FMCW) level measuring method, the microwave signal is emitted in continuous fashion but its frequency is modulated, typically by consecutive ramping. As a result of the delay during the signal propagation, the transmitter frequency us will have changed by the time the reflected signal is received back, with the frequency difference being indicative of the distance of the reflecting surface and thus of the fill level. And finally, there is the time domain reflectometry (TDR) level measuring method that is somewhat similar to the pulsed radar level measuring method but usually works via a conductor and employs electrical pulses without a carrier frequency.
A problem is encountered at times in that especially in the determination of the fill level of bulk material, the runtime of the retroreflected portion of the measuring signal does not provide a direct measure of that fill level. This is because, typically, the surface of bulk material does not form a plane but rather a cone that does not define a specific level. Moreover, the transmitted measuring signal may not even be reflected back by the medium in the container but by some device in the container such as an agitator or stirrer.
The document cited above, WO 01/11323 A1, describes a system that works at very high frequencies of several GHz, typically even more than 24 GHz. That results in an extremely narrow transmission lobe of the transmitter antenna, allowing the measuring signal to be transmitted in a specifically defined, narrow spatial direction. This also makes it possible to prevent the signal from impinging on a device in the container such as a stirrer. In fact, the point at which the transmitted measuring signal impinges on the medium in the container can be precisely selected. Nevertheless, determining the level of bulk material remains difficult due to the surface cone.